A label may have a fixed binding, which means that the label identifies (is) a
particular graph node, and different such labels are always distinct nodes.
Alternatively, a label may be unbound (variable), which means that it is a
placeholder for an unknown node label. Unbound node labels are used as
graph-local identifiers for indicating when the same node appears in
several arcs.

For the purposes of graph-isomorphism testing, fixed labels are matched when they
are the same. Variable labels may be matched with any other variable label.
Our definition of isomorphism (for RDF graphs) does not match variable labels
with fixed labels.